The Black Oil Beetle

With their slow movements, distended abdomens and their propensity for grass, oil beetles will forgive me for thinking that they are the insect equivalent of cows. Indeed for the short period that this specimen was kept in a pill bottle, it managed to eject a green liquid mass of partially digested plant bits (out of which end I did not observe) that would best be described as cud. But there is a more fascinating side to these grass-grazing beetles that belies their placid and lumbering bovinity — they secrete poison from the leg joints, the males have grappling hooks on the antennae, and the children are ruthless killers.

"The Black Oil Beetle, Meloe niger - Grasslands Provincial Park, Saskatchewan."

So bovinely sleek – The Black Oil Beetle,  Meloe niger

Oil beetles are so named because, when physically disturbed, they exude oily droplets of hemolymph from their joints. This secretion contains cantharidin, a poisonous chemical  that can cause a painful blistering (hence sometimes called ‘blister beetles’) on the skin. Certainly a handy defence when you are a fat, flightless beetle.

The lifecycle is an example of hypermetamorphosis, a variety of complete metamorphosis (holometabolism) with several distinct larval stages. After hatching, the larvae, called triungulinsactively climb plants to seek out a flower, where they will transfer to visiting bees. In Meloe franciscanus, the bees will cluster on a stem and secrete pheromones that attract male ground bees.  When the male tries to mate with the cluster, the triungulins scramble aboard. The male then moves on to mate with other real  female bees, and the triungulins can then transfer to those females. Now fertilized and loaded with larvae, she will carry the triungulins back to the nest, where the little blighters†, going through four instars, will proceed to consume the complete contents of the cell that she constructs and provisions, including the larvae. See the segment from Life in the Undergrowth below for the details of how some blister beetle triungulins  do their work. (This is probably Meloe franciscanus: see

And the grappling-hook antennae? Male oil beetles use the hooks on the antennae to latch onto the females antennae during courtship. See an image of the antennae in use at BugGuide.

"Oil beetle, male antenna hook"

Oil beetle, male antennal hook

And don’t you think this would be a good choice for Alberta’s Provincial Beetle? 😉

More oil beetle behaviour can be seen at Alberta Oil (Beetle).

For more on oil beetles see page 19 of: Alkali Bees. Their Biology and Management for Alfalfa Seed Production in the Pacific Northwest

Thanks to BugGuide for the ID

(Photographed 25 May, 2012. Grasslands National Park, Saskatchewan)

†Not an entomological term, although it should be…

Enhanced by Zemanta
This entry was posted in Attenborough, Behaviour, Canada, Coleoptera, Insect, Meloidae, National Park, Phoresy, Saskatchewan, Season, Spring, White Studio and tagged , , , , , , , , , , .


  1. Alex Wild 4 September, 2012 at 7:47 PM #

    Such an amazing insect, and a great pair of capture, Adrian!

  2. Candy Hayes 17 November, 2015 at 11:49 PM #

    Others may think this is an interesting bug, but 2 weeks ago, end of October, these nasty creatures just started showing up at my front porch and somehow finding their way into my home. I want them gone but I’m not sure how to eradicate them because I’m worried about my dog or grandchildren handling them. Any suggestions?

2 Trackbacks

  1. […] more about oil beetles and see a video of their larval development on Adrian Thysse’s blog, Splendour Awaits.) Share this:TwitterFacebookLike this:LikeBe the first to like this. This entry was posted on […]

Post a Reply to Alex Wild

Your email is never published nor shared.